a) Field of the Invention
This invention relates to a gas-shielded AC arc welding method and machine, which perform welding by supplying a substantially constant AC voltage between the consumable electrode, which is being fed at a substantially constant rate, and a workpiece.
b) Description of the Related Art
Direct current (DC) arc welding is seldom performed by straight polarity alone (hereinafter called "EN polarity"), because EN polarity tends to form a large droplet at a wire tip, an arc is unstable, large spatters are splashed, and penetration is shallow. On the other hand, reverse polarity (hereinafter called "EP polarity") features a small droplet, a stable arc and deep penetration. EP polarity has found widespread utility, since good welding results are available not only in large current welding in which an arc length is great but also in so-called short-circuiting transfer welding in which welding is performed by reducing an arc length and frequently repeating short circuiting and arcing with a small current. Further, as a gas-shielded consumable-electrode arc welding method and machine making use of an AC output, there is a technique disclosed in Japanese Patent No. 2,666,315. According to this technique, the level of a current in a period of EP polarity and the length of the period of EP polarity, which provide pinch force suitable for the smooth transfer of a droplet, are set in accordance with the material and diameter of a wire as a consumable electrode, the composition of shielding gas and the like, and the frequency of an output voltage and the level of a current in a period of EN polarity are set in relation to a wire feeding rate. This makes it possible to broaden the range of optimal welding conditions, so that rearcing is assured at alternation without superimposition of high frequency, high voltage, pulses or the like, thereby permitting stable gas-shielded AC arc welding with a consumable electrode.
As a welding machine permitting stable gas-shielded AC arc welding with a consumable electrode, the welding machine disclosed in the above-mentioned Japanese Patent No. 2,666,315 is provided with a wire feeding rate setting device for outputting a wire feeding rate setting signal in correspondence to the level of an output current, an output voltage frequency signal generator for being inputted with the wire feeding rate setting signal and then outputting an output voltage frequency signal which corresponds to a frequency of an output voltage, an EP polarity period signal generator for outputting an EP polarity period energization signal in correspondence to a setting signal set beforehand in accordance with conditions such as the material and diameter of a wire and the composition of shielding gas, an EP polarity current signal generator for outputting an EP polarity current signal in correspondence to the setting signal, an EN polarity period signal generator for outputting an EN polarity period energization signal on the basis of the EP polarity period energization signal as an input, an EN polarity current signal generator for outputting an EN polarity current signal on the basis of the wire feeding rate setting signal as an input, said wire feeding rate setting signal corresponding to the output current, an output current controller for controlling the level of an EN polarity current and that of an EP polarity current on the basis of the EN polarity current signal and the EP polarity current signal as inputs, respectively, and an output current energization period controller for controlling an EN polarity current energization period and EP polarity current energization period on the basis of the EN polarity period energization signal and EP polarity energization period as inputs, respectively, whereby a welding output is controlled.
The above-described conventional technique which makes use of an AC output is a welding method in which a droplet formed at a wire tip is transferred in a contactless manner to a base material by making use of magnetic pinch force produced by an EP polarity current of a critical current level or higher. Accordingly, the stability of an arc is impaired if the length of the arc is shortened and short circuiting is developed. Further, when the welding current level becomes higher, that is, the wire feeding rate becomes faster, the period of an EN polarity current becomes shorter, thereby making it impossible to utilize characteristic features of welding in an EN polarity period, such as shallow penetration and high wire melting rate.